In normal reciprocating piston-type machines the position of the pistons in the respective engine cylinders depends exclusively from the angular position of the crankshaft. According to a conventional arrangement for changing the compression ratio as a function of operational conditions, the connecting rod of each piston is subdivided into two connecting rod parts which are coupled with one another by a central joint. Further, a control arm is articulated at one end to the connecting rod and is secured, at its other end, to a pivotal support slidable on the machine housing.
The above-outlined constructions are described, for example, in German Offenlegungsschriften (applications published without examination) 29 35 073, 29 35 977, 30 30 615 (to which corresponds U.S. Pat. No. 4,437,438) and 37 15 391 (to which corresponds U.S. Pat. No. 4,957,069). In the structures described therein the control arm is directly coupled to the central joint, giving rise to substantial structural and operational problems. The central joint has a substantial width and thus has a large weight which, at the given spatial availabilities, cannot be compensated for by counterweights mounted on the crankshaft. On the whole, it is a disadvantage of the prior art structures that the moved masses, that is, the piston and the connecting rod, are increased and therefore greater mass forces have to be overcome.
To avoid the above-noted disadvantages, it is known to change the compression ratio by supporting the crankshaft in eccentric rings which are angularly displaceably (rotatably) supported in the cylinder block and are connected with a setting drive. By rotating the eccentric rings, the position of the crankshaft is shifted such that in the upper dead center position the pistons have a greater or lesser distance from the cylinder roof. For this purpose, German Offenlegungsschrift 30 04 402 provides that each eccentric ring is coupled with a toothed gear meshing with a pinion mounted on a setting shaft which extends parallel to the crankshaft and which is coupled with a setting drive. Apart from a substantial structural and technological input, increased space is needed for accommodating the eccentric rings and the gears disposed in their vicinity.
Further, German Offenlegungsschrift 36 01 528 describes an arrangement wherein the eccentric rings which support the crankshaft bearings are connected with a partial cylinder shell arranged concentrically to the eccentric rings and extending over the entire length of the cylinder block. The partial cylinder shell is provided on its exterior with a toothed segment which meshes with a setting worm extending transversely to the crankshaft and connected with a setting drive. Despite a favorable structural length of the crankshaft support, such a system has the disadvantage that a very compact structural component for the synchronous shifting of the eccentric rings is provided, and that because of the eccentricity of the crankshaft axis relative to the support axis of the eccentric rings, torques appearing during operation may be taken up only through the setting worm. Since in such a construction only a few teeth are in a meshing relationship with a small degree of overlap, the stress on the component materials is substantial because of the fluctuating loads generated during operation. Even a small play between the toothed segment and the setting worm may lead to a rapidly progressing wear.
Further, German Offenlegungsschrift 36 44 721 describes a system in which each eccentric ring is connected with a laterally projecting lever carrying a bearing block on its free end. Laterally of and parallel to the crankshaft a setting shaft is supported which has a setting drive and which is provided with a fork-like jaw straddling the bearing block of an eccentric ring. Since the bearing blocks cannot be guided in a play-free manner, such a system too, is disadvantageous because the fluctuating torques exerted on the eccentric rings during operation lead in this region to a significant stress on the system which is coupled with an increasing wear in the zone of the bearing block guidance.